Self-aligned source pocket for flash memory cells

ABSTRACT

An improved method for forming a flash memory is disclosed. A self-aligned source implanted pocket located underneath and around the source line junction is formed after the field oxide between adjacent word lines is removed, and before or after the self-aligned source doping is carried out, so that the configuration of the implanted boron follows the source junction profile.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor circuits and, in particular; to a method of forming self-aligned source pockets in flash memory cells.

BACKGROUND OF THE INVENTION

During the fabrication of metal-oxide-semiconductors (MOS), a silicon substrate is typically divided into a plurality of active and isolation regions by an isolation process. A thin gate oxide is subsequently grown on an upper surface of the substrate and over the active regions. A plurality of gates are then formed over the gate oxide layer, so that each gate bridges the substrate between areas to be doped as source/drain regions. The source/drain regions are consequently implanted with an impurity concentration sufficient to render them conductive.

MOS technology is greatly employed in the fabrication of non-volatile memory cells. Non-volatile memory cells are known in the art as either read-only memory (ROM) or programmable-read-only memory (PROM). One type of MOS PROM is the flash memory EEPROM (Electrically Erasable Programmable ROM).

Non-volatile MOS PROMs can be fabricated using well-known technologies such as floating gate tunnel oxide, textured polysilicon, or EEPROM-tunnel oxide, among others. The programming and erasing of the corresponding EEPROM cell differ depending upon the type of technology employed. For example, a floating gate tunnel oxide EEPROM transistor is programmed (electrons are moved into the floating gate) by biasing the control gate and drain, and erased (electrons are moved out of the floating gate) by biasing the control gate and source of the flash cell.

An example of a conventional stacked-gate flash memory cell is illustrated in FIG. 1, where on a semiconductor substrate 10, source and drain regions 12 and 14, respectively, are displaced on either side of a gate structure 30. Field oxide regions (not shown in FIG. 1) are formed by isolation techniques such as STI or LOCOS processes, and provide electrical and physical separation between neighboring source/drain active regions. A tunnel oxide 24, a floating gate 25, an inter poly dielectric (IPD) 26, and a control gate 27 form a gate structure 30 on the semiconductor substrate 10.

The operation of flash memory depends primarily on the type of techniques used to inject and/or remove charge from the floating gate 25. In general, the operation of flash memory involves (1) programming the array of memory cells, which requires a cell-by-cell control of the amount of charge stored in the floating gate; and (2) erasing the entire array, or only portions of the array, to a predetermined charge state in the floating gate. For example, some flash memories use Fowler-Nordheim tunneling both for programming as well as for erasing the memory cell array. Other flash memories, however, use hot electron injection for programming and Fowler-Nordheim tunneling for erasing.

One problem of the Fowler-Nordheim tunneling erase is the source-side depletion that occurs during a source-side Fowler-Nordheim tunneling erase operation. As known in the art, in a flash memory formed in a p-type semiconductor substrate 10 having n-type source and drain regions 12, 24, a source-side Fowler-Nordheim tunneling erase operation is obtained by applying a potential of approximately 12 volts to the source region 12, grounding the substrate 10 and setting the word line, which is connected to the control gate 27, to be at zero volts. Another bias condition is to bias the source region 12 at 6 volts and bias the control gate 27 at −10 volts. This way, an erase operation is achieved between the source region 12 and the floating gate 25, while an unwanted large voltage difference, of 12 volts or 6 volts, depending on the bias condition, is also created between the source region 12 and the substrate 10. This voltage difference further induces an undesirable substrate current as well as a hot hole current. Another problem during the erase operation is the occurrence of gate-induced diode leakage (GIDL), which also contributes to the substrate current. The substrate current can induce change in the substrate bias from about 0 volts to about 1 volts, or higher.

Attempts have been made at suppressing the unwanted current by employing a so-called double diffusion source process, which creates a gradual or two-stage change in the concentration of n-type doping between the source region 12 and the substrate 10. While this technique reduces the stress at the interface between the source region 12 and the substrate 10, suppressing therefore the unwanted current, the double diffusion source makes it difficult to decrease the size of flash memory cells.

Accordingly, there is a need for an improved flash memory with a low resistivity path during an erase operation, and no snap-backs during the programming operation. There is a further need for an improved flash memory with a reduced width for the source side depletion region that prevents extra source depletion during erase operations.

SUMMARY OF THE INVENTION

The present invention provides a method of forming a self-aligned boron implanted pocket located underneath and around the source line junction for use in flash memory and other charge storage technologies. In one exemplary embodiment of the invention, the boron implanted pocket is formed after a self-aligned source etch and a self-aligned source implant is carried out, so that the configuration of the implanted boron follows the source junction profile. In another exemplary embodiment of the invention, the source implantation is carried out after the formation of the self-aligned boron implanted pocket, while the boron pocket implant is carried out after the self-aligned source etch.

These and other features and advantages will be more clearly understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagrammatic cross-sectional view of a conventional flash memory cell.

FIG. 2 illustrates a partial top view of a semiconductor topography, at an intermediate stage of processing, wherein a flash memory cell will be constructed in accordance with the present invention.

FIG. 3 illustrates a partial top view of the representative flash memory cell of FIG. 2 at a stage of processing subsequent to that shown in FIG. 2.

FIG. 4 illustrates a partial top view of the representative flash memory cell of FIG. 2 at a stage of processing subsequent to that shown in FIG. 3.

FIG. 5 illustrates a partial top view of the representative flash memory cell of FIG. 2 at a stage of processing subsequent to that shown in FIG. 4.

FIG. 6 illustrates a cross-sectional view of the representative flash memory cell of FIG. 5.

FIG. 7 illustrates a cross-sectional view of the representative flash memory cell of FIG. 5 at a stage of processing subsequent to that shown in FIG. 6.

FIG. 8 illustrates a cross-sectional view of the representative flash memory cell of FIG. 5 at a stage of processing shown in FIG. 7.

FIG. 9 illustrates a partial top view of the representative flash memory cell of FIG. 8.

FIG. 10 illustrates a cross-sectional view of the representative flash memory cell of the FIG. 9 taken along line 10-10′ at a stage of processing subsequent to that shown in FIG. 9.

FIG. 11 illustrates a cross-sectional view of the representative flash memory cell of the FIG. 10 at a stage of processing subsequent to that shown in FIG. 10.

FIG. 12 illustrates a cross-sectional view of the representative flash memory cell of FIG. 10 at a stage of processing subsequent to that shown in FIG. 11.

FIG. 13 illustrates a partial top view of the representative flash memory cell of FIG. 12.

FIG. 14 illustrates a cross-sectional view of the representative flash memory cell of FIG. 13, taken along line 14-14′ at a stage of processing subsequent to that shown in FIG. 13.

FIG. 15 illustrates a cross-sectional view of the representative flash memory cell of FIG. 14 at a stage of processing subsequent to that shown in FIG. 14.

FIG. 16 illustrates a cross-sectional view of the representative flash memory cell of FIG. 14 at a stage of processing subsequent to that shown in FIG. 15.

FIG. 17 illustrates the topography of the source junction and that of the self-aligned boron implanted pocket of the representative flash memory cell of FIG. 16.

FIG. 18 is a schematic diagram of a processor system incorporating a flash memory cell of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, reference is made to various specific exemplary embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that structural, logical, and electrical changes may be made.

The terms “wafer” or “substrate” used in the following description may include any semiconductor-based structure that has an exposed semiconductor surface. Wafer and structure must be understood to include silicon-on insulator (SOI), silicon-on sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. The semiconductor need not be silicon-based. The semiconductor could be silicon-germanium, germanium, or gallium arsenide.

The present invention provides a method of forming a self-aligned boron implanted pocket located underneath and around the source line junction for use in flash and other charge storage technologies.

Referring now to the drawings, where like elements are designated by like reference numerals, FIG. 2 shows a partial top cross-sectional view of a flash memory array construction at an intermediate stage of processing. On a semiconductor substrate 50 having a doped well 33, a blanket sacrificial nitride layer 42 is formed by a low pressure chemical vapor deposition (LPCVD) process, for example, to a thickness of approximately 1,000 to 3,000 Å, preferably of about 1,500 Å. The sacrificial nitride layer 42 is then patterned and etched to form nitride regions 43 (FIG. 3), which protect portions of the semiconductor substrate 50 where drain and source active regions will be eventually formed leaving exposed regions 45 (FIG. 3) of the semiconductor substrate 50.

Referring now to FIG. 4, a high-temperature thermal oxidation step is performed so that field oxide regions 99 are formed on the exposed regions 45 of the semiconductor substrate 50. During the oxidation, the nitride regions 43 act as oxidation mask for active regions 88 (FIG. 5) where drain and source regions will be later formed as explained in more detail below.

Nitride regions 43 are next removed (FIG. 5) and word lines 77 are next formed (FIGS. 8-16) as gate stacks in a direction perpendicular to the longitudinal direction of the active regions 88 and field oxide regions 99. For simplicity, the formation of the word lines 77 will be explained with reference to FIGS. 6-8, which represent only cross-sectional views of the semiconductor substrate 50 of FIG. 5.

To form the word lines 77, a gate dielectric layer 52 (FIG. 6) is first formed on the semiconductor substrate 50, including the active regions 88 as well as the upper surfaces of the field oxide regions 99. The thickness of the gate dielectric layer 52 is in the range of approximately 50 to 1,000 Å, preferably of about 100 Å, values that allow the gate dielectric layer 52 to function as a tunnel oxide in the subsequent flash memory device. The value of the dielectric constant of the gate dielectric layer 52 is in the range of approximately 3.8 to 4.2.

In an exemplary embodiment, the gate dielectric layer 52 may be formed by thermal oxidation, in which the semiconductor substrate 50 is disposed in a furnace chamber maintained at a temperature of approximately 800 to 1000° C. under an oxygen ambient. Alternatively, gate dielectric 52 may be formed by thermally growing oxynitride on the substrate 50 using a nitrogen and oxygen gas.

Next, as shown in FIG. 7, a floating gate layer 54 formed of heavily doped polysilicon is deposited over gate dielectric layer 52. The preferred process for the formation of floating gate 54 includes the thermal decomposition of silane, in a CVD reactor chamber maintained at a temperature in the range of approximately 550 to 650° C. and at a pressure of less than approximately 2 Torrs. The thickness of the floating gate 54 is approximately 200 to 2,000 Å. Subsequent to the polysilicon deposition, the polysilicon layer is typically implanted with phosphorous, boron, or arsenic to lower the resistivity of the conductive polysilicon gate layer 54. Then, the floating gate layer 54 is patterned as in FIG. 7.

Next, an inter poly dielectric (IPD) layer 55 formed of SiO₂—SiN₄—SiO₂ sandwich or aluminum oxide Al₂O₃ is deposited over the floating gate 54, as shown in FIG. 8.

Referring still to FIG. 8, a control gate layer 56 is formed over the inter poly dielectric layer 55 using CVD of polysilicon. The control gate polysilicon layer 56 is preferably deposited in a CVD reactor chamber maintained at a temperature of approximately 550 to 650° C. and at a pressure of less than 2 Torrs. The thickness of the control gate layer 56 is approximately 1,000 to 2,000 Å. Dopants such as phosphorous or boron may be implanted subsequently into the control gate layer 56 to increase its conductivity.

Subsequent to the formation of control gate layer 56, conventional photolithography steps are performed so that the gate dielectric layer 52, floating gate layer 54, inter poly dielectric layer 55 and control gate layer 56 are patterned by masking the gate layers with photoresist and etching exposed portions of the gate layers to obtain word lines 77 as illustrated in FIG. 9.

Each of the two word lines 77 illustrated in FIG. 9 includes a gate dielectric 52, floating gate 54, inter poly dielectric 55 and control gate 56. FIG. 9 also shows a simplified top view of a flash memory array including a plurality of columns of active areas 88 where source and drain regions will be formed, perpendicularly intersecting a plurality of rows of word lines 77. As shown in FIG. 9, drain (D) and source (S) regions will be formed within the active areas 88 and in between the word lines 77. Field oxide regions 99 reside below the word lines 77 and around the active areas 88, thus chemically and physically isolating neighboring active areas.

Reference is now made to FIG. 10, which illustrates a cross-sectional view of the flash memory cell array of FIG. 9, taken along line 10-10′ and illustrating the two word lines 77 overlying the field oxide 99. A photoresist layer 78 is formed overlying the field oxide 99 and the word lines 77. The photoresist layer 78 of FIG. 10 is patterned by known photolithography methods to expose regions 99 a of the field oxide 99 located in between adjacent word lines 77, as shown in FIG. 11.

Regions 99 a of the field oxide 99 located in between the two adjacent word lines 77 are then etched to expose regions 99 b of the substrate 50, as shown in FIG. 12. The surface of the substrate 50 at regions 99 a is lower than the surface of the substrate 50 at the active regions 88 due to the removal of the oxide regions 99 a. For a better understanding of the removal of the oxide regions 99 a, reference is made to FIG. 13 which shows a top view of the semiconductor substrate 50, without the patterned photoresist layer 78, on which now exposed regions 99 b between adjacent word lines of the substrate 50 are better illustrated. Removal of regions 99 a from in between adjacent word lines 77 may be accomplished, for example, by selectively etching the oxide material of the field oxide 99 relative to the word lines 77 and the substrate 50 by any conventional etch chemistry.

Following the removal of the field oxide 99 from in between adjacent word lines, the photoresist 78 used for field oxide 99 etch can be used as a mask for a source region implantation. To better illustrate the formation of the source regions S, reference is made to FIG. 14, which is a cross-sectional view of the flash memory cell array of FIG. 13, taken along line 14-14′ and showing a patterned photoresist layer 78 used to etch the field oxide 99. FIG. 14 illustrates the exposed active regions 88 which will undergo self-aligned source region implantation steps within the well 33 of the semiconductor substrate 50. The well 33 is typically doped to a predetermined conductivity, for example, p-type or n-type, depending on whether N-channel or P-channel flash will be formed therein. For N-channel flash, for example, n-type doped source region 35, doped with arsenic (As) or phosphorous (P), is formed in the p-type well 33 (FIG. 15).

Referring now to FIG. 16, subsequent to the source region 35 implantation step described above, a self-aligned boron implant is performed to create a self-aligned implanted boron pocket 36. The implantation energy for the boron implantation must be higher than the implantation energy of the previous source implant, so that the higher implantation energy will lead to a deeper boron pocket 36 formed below and/or around the source region 35.

Boron implantation may be achieved with an ion source. As known in the art, the amount of the boron implant, the concentration and its distribution profile can be controlled by varying the beam current, voltage and exposure time, as well as the incidence angle of the boron ions at the surface of the substrate 50. For example, boron ions can be implanted into 0.2 to 1.0 μm areas, at an energy of approximately 30KeV and a dose of about 2×10¹²/cm² to 1×10¹⁴/cm^(2,) using a focussed ion beam implanter such as the one manufactured by Ion Beam Systems of Beverly, Mass., to form the self-aligned implanted boron pocket 36 shown in FIG. 16. This way, the boron implantation will follow the source junction profile.

Further, the boron implantation may be carried out at an incident perpendicular angle to the surface of the substrate 50. However, the boron implantation may be also carried out at incident angles that are not perpendicular to the plane of the substrate 50. Further, the boron implantation may be carried out at a 90 degree angle implant in addition to a tilt implant. By varying the incident angle at which the boron implantation is performed, the geometry of the self-aligned implanted boron pocket 36 is controlled more tightly.

The self-aligned implanted boron pocket 36 formed underneath and around the source junction follows the source junction profile, as shown in FIG. 17. FIG. 17 is a cross sectional view of the flash memory cell array of FIG. 13, taken along line 17-17′and after the formation of the self-aligned implanted boron pocket 36. As shown in FIG. 17, contour line 35 a illustrates the profile of the doping of the source region 35 of FIG. 16. Contour line 36 a, exemplifying the topography of the self-aligned implanted boron pocket 36 of FIG. 16, follows the contour line 35 a, which is the source junction profile.

Although the invention has been described with reference to forming a source implant followed by a boron implant, the boron implant can be carried out first, followed by the source implant.

Following the formation of the self-aligned implanted boron pocket 36, further processing steps may be carried out in accordance with conventional IC fabrication processes to create a functional flash memory cell. For example, photoresist layer 78 is removed and another photoresist layer is applied and patterned to expose portions of the active regions 88 where drains are to be implanted and then a drain implant is conducted. Other processing steps may include formation of voltage conductors coupled to source and drain regions 35, 37 and other known processing steps to form flash memory cell and connect them to peripheral logical devices.

The charging of the floating gate 54 to program the cell 100 is achieved by grounding source and substrate regions 35 and 33, and applying a relatively high voltage to control gate 56 and a high voltage to drain region 37. Discharge of the floating gate 54 to erase the flash memory cell is achieved by grounding control gate 56, or biasing the control gate 56 with a negative voltage, floating drain region 37, and applying a relatively high voltage to source region 35.

The resulting non-volatile memory cell 100 with the self-aligned implanted boron pocket 36 of the present invention may be utilized in a processor-based system. FIG. 18 illustrates a typical processor-based system 400 which includes a memory circuit 448, containing flash memory cells having self-aligned implanted boron pockets constructed in accordance with the present invention. A processor system 400, such as a computer system, generally comprises a central processing unit (CPU) 444, such as a microprocessor, a digital signal processor, or other programmable digital logic devices, which communicates with an input/output (I/O) device 446 over a bus 452.

In the case of a computer system, the processor system 400 may include peripheral devices such as a floppy disk drive 454 and a compact disk (CD) ROM drive 456, which also communicate with the CPU 444 over the bus 452. Memory 448 is preferably constructed as an integrated circuit, which includes flash memory cells having self-aligned implanted boron pockets in the same regions formed as previously described with respect to FIGS. 2-17. The memory 448 may be combined with the processor, for example CPU 444, in a single integrated circuit.

Although the exemplary embodiments described above refer to one self-aligned boron implanted pocket, it must be understood that the present invention contemplates the use of a plurality of self-aligned boron implanted pockets formed beneath source regions so that the configuration of the implanted boron follows the source junction profile. Further, although the exemplary embodiments described above refer to the formation of the self-aligned boron implanted pocket after the source implant is carried out, it must be understood that the invention also contemplates the formation of the boron implanted pockets before the source implantation, as long as the field oxide has been previously removed from in between adjacent word lines.

Although the embodiments described above refer to the formation of a pocket implanted with dopant atoms, such as boron, it must be understood that the invention is not limited to the use of boron and other p-type dopants may be used also, for example BF₂ Furthermore, p-type dopants, such as boron (B), may be used also if an n-well is formed in place of p-well 33 over the substrate 50, with of course, the doping of the relevant active area regions changing accordingly. In addition, although the embodiments described above refer to the formation of a region implanted with dopant atoms in the form of a pocket, it must be understood that the invention is not limited to the above configuration, and other geometries, such as a round shape, may be employed also, as long as the dopant implantation is carried out subsequent to the field oxide removal.

Accordingly, the above description and drawings are only to be considered of illustrative embodiments which achieve the features and advantages of the present invention. Modifications and substitutions to specific process conditions and structures can be made without departing from the spirit and scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description and drawings, but is only limited by the scope of the appended claims. 

What is claimed as new and desired to be protected by Letters Patent of the United States is:
 1. A method of forming a plurality of dopant pockets on a substrate comprising: forming a plurality of implantable regions on said substrate separated by field oxide regions, said implantable regions and field oxide regions extending in a first direction; forming a plurality of word lines located over said implantable regions and field oxide regions, said word lines extending in a second direction perpendicular to said first direction; removing portions of said field oxide regions between two adjacent word lines to expose respective substrate regions; forming source regions of a first conductivity type in said implantable regions; and subsequently implanting a dopant of a second conductivity type which is different from said first conductivity type into said substrate through said respective substrate regions to form said dopant pockets beneath said source regions.
 2. The method of claim 1, wherein said dopant is a p-type dopant.
 3. The method of claim 1, wherein said dopant is boron.
 4. The method of claim 1, wherein said dopant is BF₂.
 5. The method of claim 1, wherein said dopant is an n-type dopant.
 6. The method of claim 1, wherein said act of removing portions of said field oxide regions further comprises selectively etching said portions of field oxide regions relative to said substrate and said word lines.
 7. The method of claim 1, wherein each of said word lines is formed of a gate stack comprising a gate oxide, a floating gate, a dielectric formed over said floating gate, and a control gate formed over said dielectric.
 8. The method of claim 1, wherein said act of implanting said dopant is carried out with an implanting energy higher than implanting energy used to form said source regions.
 9. The method of claim 8, wherein said act of implanting said dopant employs directing said dopant through said substrate region at an angle of substantially 90 degrees incidence to said substrate region.
 10. The method of claim 8, wherein said act of implanting said dopant employs directing said dopant through said substrate region at angles different than substantially 90 degrees incidence to said substrate region.
 11. A method of forming a plurality of dopant pockets on a substrate comprising: forming a plurality of implantable regions on said substrate separated by field oxide regions, said implantable regions and field oxide regions extending in a first direction; forming a plurality of word lines located over said implantable regions and field oxide regions, said word lines extending in a second direction perpendicular to said first direction; forming a layer of photoresist over said field oxide regions and over said plurality of word lines; patterning said photoresist to expose portions of said field oxide regions between said word lines; removing portions of said field oxide regions between adjacent word lines to expose respective substrate regions; forming source regions in said implantable regions; and implanting a dopant into said substrate through said respective substrate regions to form said dopant pockets beneath said source regions, said photoresist operating as a mask for forming said source regions and said dopant pockets, wherein said act of implanting said dopant into said substrate is carried out before said act of forming said source regions.
 12. A method of forming source regions with boron pockets on a substrate of a flash memory, said method comprising: forming a field oxide layer over said substrate; forming a pair of adjacent spaced word lines over said field oxide layer; removing said field oxide layer from predefined regions located in between said spaced word lines to expose respective substrate regions; forming a source region in between said word lines; and subsequently implanting boron into said substrate in between said word lines to form a boron pocket beneath said source region.
 13. The method of claim 12, wherein said act of implanting boron into said substrate is carried out after said act of removing said field oxide material.
 14. The method of claim 12, wherein said act of implanting boron is carried out at with an implanting energy higher than an implanting energy for said source region.
 15. The method of claim 14, wherein said act of implanting boron employs directing boron through said substrate region at an angle of substantially 90 degrees incidence to said substrate region.
 16. The method of claim 14, wherein said act of implanting boron employs directing boron through said substrate region at angles different than substantially 90 degrees incidence to said substrate region.
 17. A method of forming source regions with boron pockets on a substrate of a flash memory, said method comprising: forming a field oxide layer over said substrate; forming a pair of adjacent spaced word lines over said field oxide layer; forming a layer of photoresist over said field oxide layer and over said word lines; patterning said photoresist to expose a portion of said field oxide layer located between said word lines; removing said field oxide layer from predefined regions located in between said spaced word lines to expose respective substrate regions; forming a source region in between said word lines; and implanting boron into said substrate in between said word lines to form a boron pocket beneath said source region, said photoresist operating as a mask for forming said source region and said boron pocket, wherein the act of implanting boron into said substrate is carried out before said act of forming said source region.
 18. A method of forming a source region in a substrate comprising: forming a pair of gate structures which extend in a first direction over a substrate; altering the upper surface profile of said substrate to form alternating areas of higher substrate surface elevation and areas of lower substrate surface elevation along said first direction and between said pair of gate structures; providing a first doped layer in said substrate between said gate structures which has a profile which follows that of said upper surface profile; and providing a second doped layer in said substrate between said gate structure which is below said first doped layer and which has a profile which follows that of said first doped layer, wherein said act of providing said second doped layer is carried out with an implanting energy higher than an implanting energy for said first doped layer.
 19. The method of claim 18, wherein at least one of said areas of higher and lower substrate surface elevation is doped by said first doped layer to act as a source region of a transistor.
 20. The method of claim 19, wherein said area of higher substrate surface elevation acts as said source region.
 21. A method of forming a source region in a substrate comprising: forming a pair of gate structures which extend in a first direction over a substrate; altering the upper surface profile of said substrate to form alternating areas of higher substrate surface elevation and areas of lower substrate surface elevation along said first direction and between said pair of gate structures; providing a layer of photoresist over said pair of gate structures; patterning said photoresist to expose a portion of said substrate located between said pair of gate structures; providing a first doped layer in said substrate between said gate structures which has a profile which follows that of said upper surface profile; and providing a second doped layer in said substrate between said pair of gate structures which is below said first doped layer and which has a profile which follows that of said first doped layer, said photoresist operating as a mask for providing said first doped layer and said second doped layer, wherein said act of providing said second doped layer is carried out with an implanting energy higher than an implanting energy for said first doped layer and wherein said second doped layer is provided in said substrate before said first doped layer. 